This invention relates to methods and systems for providing emission of incoherent radiation and uses for therefor.
Currently, commercial dielectric barrier discharge (DBD) lamp sources of incoherent ultraviolet (UV) are inherently low-peak power and are poorly suited to many practical applications Alternative sources of high-peak power UV radiation (laser-based) are comparatively high-cost and not cost-effective for many desired industrial processes. Dielectric barrier discharge lamps used to generate ultraviolet output generally employ electrical excitation schemes based on an AC voltage waveform (50 Hz-200 kHz). Although the UV emitted by the plasma can be generated with high efficiency (xcx9c10-20%) and with high average power, the present inventors have realised that the UV output has inherently low-peak power due to the dynamics of the plasma excitation when using AC excitation.
It is an object of this invention to provide methods and systems for providing emission of incoherent radiation and uses therefor.
According to a first embodiment of this invention there is provided a method of operating a system for providing emission of incoherent radiation, said system comprising an electrically impeded discharge lamp linked to an electrical energy supply, said lamp comprising:
(a) a discharge chamber which is at least partially transparent to said incoherent radiation;
(b) a discharge gas in said chamber;
(c) two electrodes disposed with respect to said chamber for discharging electrical energy there between;
(d) at least one dielectric barrier disposed between said two electrodes to electrically impede electrical energy passing between said two electrodes;
(e) an electrical energy supply capable of providing fast risetime unipolar voltage pulses;
(f) means of electrically linking said electrodes with said supply; said method comprising:
providing a sequence of unipolar voltage pulses from said energy supply to said electrodes and controlling (i) interpulse period, an (ii) pulse risetime, whereby a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation from said lamp.
According to a second embodiment of this invention there is provided a method of operating a system for providing emission of high peak power incoherent radiation, said system comprising an electrically impeded discharge lamp linked to an electrical energy supply, said lamp comprising:
(a) a discharge chamber which is at least partially transparent to said incoherent radiation;
(b) a discharge gas in said chamber;
(c) two electrodes disposed with respect to said chamber for discharging electrical energy there between;
(d) at least one dielectric barrier disposed between said two electrodes to electrically impede electrical energy passing between said two electrodes;
(e) an electrical energy supply capable of providing fast risetime, high peak unipolar voltage pulses;
(f) means of electrically linking said electrodes with said energy supply; said method comprising;
providing a sequence of high peak power unipolar voltage pulses from said energy supply to said electrodes and controlling (i) interpulse period, and (ii) pulse risetime, whereby a substantially homogeneous discharge occurs between said two electrodes which causes emission of incoherent radiation pulses of high peak power from said lamp.
According to a third embodiment of this invention there is provided a system for providing emission of incoherent radiation, said system comprising an electrically impeded discharge lamp linked to an electrical energy supply, said lamp comprising:
(a) a discharge chamber which is at least partially transparent to said incoherent radiation;
(b) a discharge gas in said chamber;
(c) two electrodes disposed with respect to said chamber for discharging electrical energy there between;
(d) at least one dielectric barrier disposed between said two electrodes to electrically impede electrical energy passing between said two electrodes;
(e) an electrical energy supply capable of providing fast risetime unipolar voltage pulses;
(f) means of electrically linking said electrodes with said energy supply;
said energy power supply being capable of providing a sequence of unipolar voltage pulses from said energy supply to said electrodes; and
means to control (i) interpulse period, and (ii) pulse risetime, whereby, in use, a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation from said lamp.
According to a fourth embodiment of this invention there is provided a system for providing emission of high peak power (in watts) incoherent radiation, said system comprising an electrically impeded discharge lamp linked to an electrical energy supply, said lamp comprising:
(a) a discharge chamber which is at least partially transparent to said incoherent radiation;
(b) a discharge gas in said chamber;
(c) two electrodes disposed with respect to said chamber for discharging electrical energy there between;
(d) at least one dielectric barrier disposed between said two electrodes to electrically impede electrical energy passing between said two electrodes;
(e) an electrical energy supply capable of providing fast risetime, high peak unipolar voltage pulses;
(f) means of electrically linking said electrodes with said supply;
said energy supply being capable of providing a sequence of high peak unipolar voltage pulses from said energy supply to said electrodes; and
means to control (i) interpulse period, and (ii) pulse risetime, whereby, in use, a substantially homogeneous discharge occurs between said two electrodes which causes emission of incoherent radiation pulses of high peak power from said lamp.
Other embodiments of the invention include:
(1) a method of releasing contaminants from a surface by irradiating the surface with incoherent radiation pulses generated by a method of the invention, said pulses being of sufficient intensity (W/cm2) to release said contaminants from said surface;
(2) a method of modifying a surface by irradiating the surface with incoherent radiation pulses generated by a method of the invention, said pulses being of sufficient intensity to modify said surface;
(3) a method of ablating/etching a material by irradiating the material with incoherent radiation pulses generated by a method of the invention, said pulses being of sufficient intensity to ablate/etch said surface;
(4) a method of pumping a laser active medium by irradiating the active medium with incoherent radiation pulses generated by a method of the invention, said pulses being of sufficient intensity to pump said active medium;
(5) a method of killing micro-organisms and/or bacteria by irradiating the bacteria with incoherent radiation pulses generated by a method of the invention, said pulses being of sufficient intensity to kill said micro-organisms and/or bacteria;
(6) a method of irradiating an object with incoherent radiation pulses generated by a method of the invention, comprising irradiating said object with said pulses;
(7) a method of removing surface contaminants by irradiating the surface with incoherent radiation pulses generated by a method of the invention, comprising irradiating said surface with said pulses using various methods to achieve inert gas flow over the irradiated surface, said pulses being of sufficient intensity to remove said surface contaminants (see U.S. Pat. No. 5,821,175 for methods to achieve inert gas flow over the irradiated surface);
(8) a method of controlling insects and/or mites by irradiating the insects and/or mites with incoherent radiation pulses generated by a method of the invention, said pulses being of sufficient intensity to kill said insects and/or mites;
(9) a system for releasing contaminants from a surface said system being capable of irradiating the surface with incoherent radiation pulses, said pulses being of sufficient intensity to release said contaminants from said surface;
(10) a system for modifying a surface said system being capable of irradiating the surface with incoherent radiation pulses, said pulses being of sufficient intensity to modify said surface;
(11) a system for ablating/etching a material said system being capable of irradiating the material with incoherent radiation pulses, said pulses being of sufficient intensity to ablate/etch said surface;
(12) a system for pumping a laser active medium said system being capable of irradiating the medium with incoherent radiation pulses, said pulses being of sufficient intensity to pump said active medium;
(13) a system for killing micro-organisms and/or bacteria said system being capable of irradiating the bacteria with incoherent radiation pulses, said pulses being of sufficient intensity to kill said micro-organisms and/or bacteria;
(14) a system of removing surface contaminants said system being capable of irradiating the surface with incoherent radiation pulses, said pulses being of sufficient intensity to remove said surface contaminants;
(15) a system of controlling or killing insects and/or mites said system being capable of irradiating the insects and/or mites with incoherent radiation pulses, said pulses being of sufficient intensity to control or kill said insects and/or mites;
Typically the two electrodes are disposed in the chamber.
The methods of the invention usually comprise:
providing a sequence of unipolar voltage pulses from said energy supply to said electrodes, and controlling (i) interpulse period, (ii) pulse risetime, and (iii) pulse width, whereby a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation from said lamp.
The methods of the invention may comprise:
providing a sequence of unipolar voltage pulses from said energy supply to said electrodes and controlling (i) interpulse period, (ii) pulse risetime, (iii) pulse width, (iv) interpulse voltage level, and (v) unipolar pulse voltage level; whereby a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation from said lamp.
The systems of the invention usually comprise:
means to control (i) interpulse period, (ii) pulse risetime, and (iii) pulse width, whereby, in use, a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation from said lamp.
The systems of the invention may comprise:
means to control (i) interpulse period, (ii) pulse risetime, (iii) pulse width, (iv) interpulse voltage level, and (v) unipolar pulse voltage level; whereby, in use, a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation from said lamp.
More typically the high peak power methods of the invention comprise:
providing a sequence of unipolar voltage pulses from said energy supply to said electrodes and controlling (i) interpulse period, (ii) pulse risetime, and (iii) pulse width, whereby a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation of high peak power from said lamp.
The high peak power methods of the invention may comprise:
providing a sequence of unipolar voltage pulses from said energy supply to said electrodes and controlling (i) interpulse period, (ii) pulse risetime, (iii) pulse width, (iv) interpulse voltage level, and (v) unipolar pulse voltage level; whereby a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation of high peak power from said lamp.
More typically the high peak power systems of the invention comprise:
means to control (i) interpulse period, (ii) pulse risetime, and (iii) pulse width, whereby, in use, a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation of high peak power from said lamp.
The high peak power systems of the invention may comprise:
means to control (i) interpulse period, (ii) pulse risetime, (iii) pulse width, (iv) interpulse voltage level, and (v) unipolar pulse voltage level; whereby, in use, a substantially homogeneous discharge occurs between said two electrodes which causes emission of pulses of incoherent radiation of high peak power from said lamp.
The chamber may have a discharge gas inlet and a discharge gas outlet. The discharge gas pump may be linked to the chamber to either increase or reduce and/or provide discharge gas to the chamber. A supply of discharge gas may be linked to the chamber.
At high peak power, one pulse of UV/VUV emission is observed following the application of each unipolar voltage pulse and passage of the associated discharge current pulse. At high peak power the output of the discharge chamber comprises high output pulse energy (in joules) (within xcx9c20%, more usually within xcx9c10% of the maximum output pulse energy) and small output pulse width (in nanoseconds) (within xcx9c20%, more usually within xcx9c10% of minimum output pulse width). Usually to generate UV/VUV output with high peak power characteristics, the specific operating conditions of the discharge chamber or lamp should be selected so as to substantially maximise the output pulse energy (in joules) and substantially minimise the output pulse width (in nanoseconds). By monitoring a typical UV/VUV pulse emitted by the lamp of known (fixed) surface area, high peak power operation can be characterised by measuring the instantaneous peak output power (in watts) which should be substantially maximised in amplitude.
The systems and/or methods of the invention may include means to control the amplitude of the unipolar voltage pulses, means to control pressure and/or temperature of said discharge gas, and means to control pulse width.
The systems and/or methods of the invention may include means to adjust the amplitude of the unipolar voltage pulses (e.g. an adjustable power supply), means to adjustably control gas pressure in the discharge chamber (e.g. via an adjustable gas pressure supply to the discharge chamber) and/or temperature of said discharge gas (e.g. via an adjustable temperature controller to a heat element coupled or operably associated with the discharge chamber), means to adjustably control pulse interpulse period (e.g. an adjustable power supply), means to adjustably control pulse width (e.g. via an adjustable power supply), means to adjustably control interpulse voltage level (e.g. via an adjustable power supply), and/or means to adjustably control pulse risetime (e.g. via an adjustable power supply).
The systems and/or methods of the invention may include means to detect the amplitude of the unipolar voltage pulses (e.g. an oscilloscope or voltmeter), means to detect pressure (e.g. a pressure gauge) and/or temperature (e.g. a thermocouple linked to appropriate electronics) of said discharge gas, means to detect interpulse period (e.g. an oscilloscope or voltmeter), means to detect pulse width, amplitude and/or means to detect pulse risetime (e.g. an oscilloscope means to detect interpulse voltage level (e.g. an oscilloscope or voltmeter), and/or means to detect discharge current (e.g. an oscilloscope or ammeter).
The systems and/or methods of the invention may include means to trigger the energy pulse.
The systems and/or methods of the invention may include means to monitor the amplitude of the unipolar voltage pulses pulses (e.g. an oscilloscope or voltmeter), means to monitor pressure (e.g. a pressure gauge or a pressure detector linked to appropriate electronics) and/or temperature (e.g. a thermocouple linked to appropriate electronics) of said discharge gas, means to monitor pulse idle time pulses (e.g. an oscilloscope or voltmeter), means to monitor pulse width pulses (e.g. an oscilloscope), and/or means to monitor pulse risetime (e.g. an oscilloscope), and/or means to monitor discharge current (e.g. an ammeter).
The systems and the methods of the invention may include means to adjust the composition of the discharge gas.
The systems,and methods of the invention may include means to detect the emission of incoherent radiation pulses. The systems and methods of the invention may include means to detect the emission of incoherent radiation pulses and to measure the intensity of the pulses.
The systems and methods of the invention may include means to focus the emitted incoherent light.
The embodiments of the invention provide methods of and systems for generating light usually ultraviolet light or vacuum ultraviolet light from dielectric barrier discharges (DBD). The methods generate and the systems are capable of generating UV or VUV pulses of short duration (100-500 ns) and, where required, high-peak power UV or VUV pulses. This has been made possible through the use of electrical circuits, which supply single-pulse voltage waveforms of short duration (typically up to 5 xcexcs, more typically tip to 1 xcexcs) and operating procedures to xe2x80x9csynchronisexe2x80x9d excitation of the plasma throughout the volume of the lamp resulting in a homogeneous discharge. The excitation pulses from the circuit are separated by relatively long xe2x80x9cidlexe2x80x9d or xe2x80x9coffxe2x80x9d periods, typically in the range 5-2000 xcexcs (or 500 Hz-200 kHz), 5-1000 xcexcs, 5-1500 xcexcs, 5-750 xcexcs, 5-500 xcexcs, 5-250 s, 5-100xcexcs, 250-800 xcexcs, 275-800 xcexcs. 275-700 xcexcs, 275-600 xcexcs. 275-500 xcexcs, 275-400 xcexcs, 275-350 xcexcs, 275-325 xcexcs, where the applied voltage is set to zero and where no plasma excitation occurs in the discharge chamber or a value other than 0 volts and where no plasma excitation occurs discharge chamber. Typically, excitation pulses from the circuit are separated by relatively long xe2x80x9cidlexe2x80x9d or xe2x80x9coffxe2x80x9d periods, of 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 425, 450, 475, 500, 550, 600, 650, 700, 750, 800, 1000, 1250, 1500, 1750 or 2000 microseconds.
The amplitude of the unipolar voltage pulses is dependent on lamp geometry and required output but is usually between 0.5 kV-70 kV, 3 kV-50 kV, or 5 kV-30 kV, 5 kV-25 kV, more usually between 5 kV-20 kV, 5 kV-17 kV, 5 kV-16 kV, 5 kV-15 kV, 6 kV-15 kV, 6 kV-14 kV and even more typically between 6 kV-13 kV. The amplitude of the unipolar voltage pulses may be, for example, 1 kV, 2 kV, 3 kV, 4 kV, 5 kV, 6 kV, 7 kV, 8 kv, 9 kV, 10 kV, 11 kV, 12 kV, 13 kV, 14 kV, 15 kV, 16 kV, 17 kV, 18 kV, 19 kV, 20 kV, 25 kV, 30 kV, 35 kV, 40 kV, 45 kV, 50 kV, 55 kV, 60 kV, 65 kV or 70 kV. Usually the amplitudes of the unipolar voltage pulses are less than about 16 kV. The amplitude of each of the unipolar voltage pulses may be the same or different.
The voltage waveform pulse duration is typically in the range 0.05 to 5, 0.1 to 4, 0.1 to 3, 0.1 to 2.5, 0.1 to 2, 0.1 to 1.75, 0.1 to 1.5, 0.1 to 1.25, 0.1 to 1, 0.1 to 0.75, 0.1 to 0.5, 0.5 to 1.5, 0.5 to 1.25, 0.5 to 1, 0.5 to 0.75, 0.75 to 1.5, 0.75 to 1.25, 0.75 to 1, 1 to 1.5, 1 to 2, or 0.9 to 1.1 microseconds. The pulse duration is typically 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3, 3.5, 4.0, 4.5 or 5.0 microseconds.
Usually the interpulse voltage level is 0 volts or at a voltage level whereby no discharge occurs between the two electrodes in the system. More usually the interpulse voltage level is 0 volts or at a voltage level which is substantially below the voltage level whereby a discharge occurs between the two electrodes in the system (typically in the range between 0 volts up to 75%, 0 volts up to 50%, 0 volts up to 25%, 0 volts up to 10% or 0 volts up to 5% of the voltage level whereby a discharge occurs between the two electrodes in the system).
As well as optimising the excitation circuitry for high peak power operation it has been found that higher gas pressures are needed for this new type of operation than are typical for standard DBD lamps. Typically, for high peak power operation (and for other operations, if required) the gas pressure in the discharge chamber is greater than 1 atmosphere pressure. Typically the gas pressure in the discharge chamber is in the range of from about 1.001 atmospheres-3 atmospheres, 1-5 atms, 1-3 atms, 1-2 atms. 1.001-2.5 atms, 1.001-2 atms, 1.001-1.75 atms, 1.001-1.5 atms or 1.001-1.3 atms especially for high peak power operation. The gas pressure may be below atmospheric for certain uses (for example, high efficiency operation and in some instances high peak power operation). Where the gas pressure is below or at atmospheric pressure it is typically in the range of 180 to 760 torr, more typically to 250 to 760, more typically 350 to 760, and even more typically 400 to 760 and yet even more typically 500 to 760 or 600 to 760 torr. Usually, the gas pressure in the discharge chamber for high peak power operation is 761, 762, 763, 764, 765, 766, 767, 768, 769, 770, 775, 780, 785, 790, 795, 800, 810, 820, 830, 840, 850, 875, 900, 925, 950, 975, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450, 1500, 1550, 1600, 1650, 1700, 1750, 1800, 1850, 1900, 1950, 2000, 2050, 2100, 2200, 2300, 2400 or 2500 torr.
The risetime of the voltage pulse is typically in the range of 5 to 1300, 10 to 1250, 15 to 1150, 20 to 1100, 25 to 1050, 30 to 1000, 35 to 950, 50 to 900, 75 to 850, 100 to 800, 100 to 750, 100 to 720, 100 to 700, 100 to 675, 100 to 650, 100 to 625, 100 to 600, 100 to 575, 100 to 550, 100 to 525, 100 to 500, 100 to 475, 100 to 450, 100 to 425, 100 to 400, 100 to 375, 100 to 350, 100 to 325, 100 to 300, 100 to 275, 100 to 250, 100 to 225, 100 to 200, 100 to 175, 100 to 150, 100 to 125, 125 to 350, 125 to 300, 125 to 250, 125 to 225, 125 to 200, 125 to 175, 125 to 150, 150 to 325, 1,50 to 300, 150 to 275, 150 to 250, 150 to 225, 150 to 200, 150 to 175, 175 to 325, 175 to 300, 175 to 275, 175 to 250, 175 to 225, 175 to 200, 200 to 350, 200 to 325, 200 to 300, 200 to 275, 200 to 250, 200 to 230, 200 to 225, 200 to 220, 200 to 210, 200 to 400, 200 to 350, 200 to 500, 200 to 450, 200 to 425, 210 to 400, or 220 to 250 nanoseconds. The risetime of the voltage pulse is typically 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 205, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 450, 500, 550, 600, 700, 800, 900, 1000, 1100, 1200, or 1300 nanoseconds.
The methods and systems of the invention are capable of providing a source of high-peak-power incoherent ultraviolet (UV) light (80-350 nm, more typically 11-320 nm). The high-peak-power mode of operation is made possible by the method of the invention using a short-pulse excitation scheme of a plasma lamp of the dielectric barrier discharge (DBD) type. Although there has been considerable effort worldwide in developing DBD lamp technology as efficient sources of high-average power UV over the past ten years, no attention has been directed towards operating these lamps to generate short-pulse, high-peak-power UV output. Such a source of high-peak-power UV radiation may be used for a variety of industrial applications relating to surface modification (ablation and chemical reactions) and materials processing for which processing rates are strongly dependent on the rate of UV energy density deposition and which may be characterised by a threshold fluence. This category of materials processing cannot be easily undertaken with commercial DBD lamps currently available as these operate with high-average power, but low-peak-power UV output and hence yield poor performance such as low etch rates. More commonly, laser-based sources of high-peak power UV radiation are used for such applications. Several different output wavelengths are possible from DBD lamps depending on the gas mixture used in the discharge namely, XeCl (308 nm), KrF (248 nm), KrCl (222 nm), ArCl (175 nm), XeF (354 nm), Xel (253 nm), XeBr (283 nm), Krl+ (190 nm) KrBr (207 nm), ArBr (1.65 nm), Xe2+ (172 nm), Kr2+ (146 nm) and Ar2+ (126 nm) and Ne2+ (88 nm), and He2+. The methods of the invention may be applied to provide short pulsed, high peak power output is applicable to DBD lamps based on all these gas mixtures.
The discharge gap is in the range in which a substantially homogeneous discharge can take place and be stably sustained. Usually the discharge gap is less than or equal to about 10 mm. Typically the discharge gap is in the range 0.5 to 10 mm, more typically 1.0 to 7 mm, more typically 1.5 to 5 mm, and more typically 2 to 3 mm.
To operate a dielectric barrier discharge (DBD) lamp, being a source of incoherent ultraviolet (UV) radiation, in a manner whereby the UV generated by the DBD appears in the form of single (and intense) pulses of short duration (e.g. 50-500 ns) during each cycle of the lamp excitation, these pulses constituting high peak-power UV output. The lamp geometry, operating conditions and procedures are optimised so as to maximise the peak power of the individual UV output pulses.
This mode of operation is achieved through the use of pulsed electrical excitation (in particular using voltage pulses with rapid rise times) and by optimising the lamp operating parameters so as to increase the production rate (and shorten the formation time) of the dimer molecules from which the UV radiation is derived. An important characteristic of the high-peak power operation is that the UV radiation is often generated (but not necessarily) from a spatially uniform or homogeneous discharge plasma, rather than a filamentary type (streamer) plasma more commonly associated with conventional AC excited DBD lamps. The cause of the homogeneous discharge is thought to be caused by the rapid rate at which the applied E-field reaches the necessary condition for homogeneous discharge to occur at a faster rate than the formation of filaments. It is thought that the fast application of the applied E-field to the electrodes leads to a spatially uniform electron avalanche such that the discharge breakdown is caused to occur in a homogeneous fashion.
These operating procedures could be applied in principal to existing DBD lamp configurations, which have been almost exclusively, excited by AC power supplies up until the present invention. By following the method of the invention the characteristics of the UV output from low-peak power (AC excited) usually characterised by a periodic pattern of multiple filamented (ie streamer) micro-discharges over the dielectric surface change to a substantially pale blue (in the case of UV radiation from Xenon) homogeneous (glow like) discharge (pulsed excitation) over the dielectric surface. Further, by following the high peak power method of operation disclosed herein, the dielectric barrier discharge lamp may be operated in high peak power mode (pulsed excitation).
In DBD plasma lamps utilizing a single atomic species of a noble or rare gas R, the UV emission is derived from the radiative decay of the R2* dimer molecule produced in the plasma via kinetic reactions. To obtain high peak-power UV output from such a lamp, it is necessary to ensure that the R2* dimers are generated as quickly as possible, and that the production rate is uniformly fast throughout the plasma volume. The pulse width of the UV output is then ultimately governed by (and limited by) the lifetime for radiative decay of the dimer (e.g. xcfx84xcx9c5 ns for Xe2*1xcexa3u* and xcfx84xcx9c100 ns for Xe2+ 3xcexa3u*). To this end, power must be deposited in the plasma on a timescale which must be comparable to, or faster than, the conversion time of rare gas excited states R* into dimers R2* so that the production rate (or formation time) of R2* is not limited, by formation time of excited states R*. The production rate of R2* from R* can be increased by raising the gas pressure (density of R) as in (2).
Using voltage pulses with fast rise times (e.g. xcfx84xcx9c50 ns-1000 ns, more typically 50 ns-500 ns) and optimising the lamp operating parameters, electrical power is deposited in the plasma on the requisite timescale for rapid R* production, by virtue of the single (and relatively large) current pulse of short duration (xcfx84 less than 50 ns) which is observed. (Note: in conventional AC excited DBDs, multiple discharge current pulses of relatively low amplitude are observed during the cycle of the AC voltage waveform). The total number of UV photons generated in the plasma (directly affecting, the peak power) is dependent on the number of R* species generated when power is deposited in the plasma. Thus, it is preferable to select operating conditions such that the plasma excitation for R* production is optimised and is homogeneous throughout the plasma volume. An important feature of the present invention for high peak power operation is that a homogeneously excited plasma will avoid xe2x80x9cdead-zonesxe2x80x9d of gas excitation between filament columns as found with conventional AC excited DBD""s.
Practically, the voltage pulse risetime is found to be critically important in maintaining a homogeneous discharge plasma. In fact, using very short voltage pulses permits the DBD to operate at a higher pressure than for an AC excited DBD whilst maintaining homogeneous plasma excitation. This is an important advantage of using fast voltage pulses since a higher operating pressure favours rapid conversion of R* to R2* as in (2) to achieve short pulse high-peak power UV output.
Variables that may be altered include the usual ways of optimisation of UV output xe2x80x9cpowerxe2x80x9d, increasing repetition rate raises average output power (but not peak power), using thinner dielectrics, changing xcex5 dielectric material, electrode geometry, gas pressure, electrode area, electrode spacing, interpulse period, interpulse voltage amplitude (typically at 0 volts or at a level whereby there is no lamp discharge) and initial conditions. Gases and mixtures thereof which may be utilised to provide high-peak power UV/VUV include He, Ne, Ar, Kr, Xe, F, Cl, Br, and mixtures thereof. Bipolar or other suitable voltage pulses may also be used. Any suitable lamp geometry and electrode configuration may be used including a cylindrical configuration, flat or coaxial designs, for example.
Typically, the performance of a DBD lamp is determined as a function of various discharge parameters These include buffer gas pressure, physical separation between the dielectric surfaces (cell-width), excitation peak voltage risetime of applied voltage pulse, duration of applied voltage pulse, time delay between voltage pulses (or interpulse period), interpulse voltage level (typically xcx9c0 volts). Specifically, DBD lamp performance may be monitored and assessed using the following electrical and spectroscopic measurements:
Time-resolved (a) voltage waveforms using a high-voltage probe and wide-bandwidth (500 MHz) digital oscilloscope, (b) current waveforms from the voltage drop across a series resistor;
Displaced charge through the lamp plasma by monitoring the voltage on a series capacitor;
Electrical energy deposition calculated by integrating the displaced charge with respect to the applied voltage over each complete cycle;
Examination of the voltage/charge Lissajous figures (yields useful information on the lamp electrical breakdown characteristics, and the plasma impedance in pulsed DBDs in the period corresponding to the trailing edge of the voltage pulse).
Temporal evolution of the UV/VUV output pulses (e.g. by detection on a sodium salicylate phosphor for conversion to visible wavelengths and detection by a standard photomultiplier);
Absolute UV/VUV output power measurements using a calibrated silicon pn photo-diode and optical double-aperture system to define solid-angle and lamp emission area.
Visible emission spectra 320 nm-600 nm using a 0.5 m SPEX spectrometer arid N2 purge (VUV output at 160-180 nm appears in second-order).
Time-resolved population densities of Xe* 1s5 and 1s4 low-lying levels by absorption at 462.6 nm and 492.5 nm using a frequency tripled YAG pumped dye-laser. Formation of Xe2* dimers (yielding VUV output) proceeds via the 1s5 and 1s4 levels (analogous levels for Ar and Kr and other gases may be similarly detected).
This invention provides relatively inexpensive systems and methods to generate incoherent UV/VUV light pulses whose properties (short duration, high-peak-power) can be specifically targeted at a wide range of applications including industrial materials processing. The systems of the invention provide low-cost sources of incoherent UV/VUV light covering a broad range of wavelengths, typically 110 to 320 nm. The systems and methods of the invention have the potential to replace the use of high-cost ultraviolet pulsed lasers to dramatically Improve commercial viability in some manufacturing processes. In addition the invention is expected to lead to new applications due to the low-cost UV/VUV light that the systems and methods of the invention are able to supply where the current commercial viability of the manufacturing process or applications is inhibited by the high cost of existing laser sources.
The method of the invention based on pulsed DBD lamp is applicable to a raft of surface cleaning, surface modification, moderate-threshold-ablation/etching processes and UV light assisted deposition of materials as well as being a potential optical pump source for several laser gain media and a potential means of killing micro-organisms and bacteria. Currently, short pulse laser sources (predominantly Nd:YAG at 1.06 xcexcm, KrF excimer lasers at 248 nm, frequency quadrupled Nd:YAG at 266 nm and frequency doubled copper vapour lasers at 255 nm) are employed for micromachining of materials such as polymers, metals; removal of micron and submicron sized particulates from surfaces as varied as silicon wafers, silica glass, magnetic head sliders (either with or without assistance by surface layers of water or solvents); removal of hydrocarbon (e.g. fingerprints) and other chemical contaminants from silicon, glass, metals, stone etc without removal of the base material, ablation of polymers; dehydroxylation of silica surfaces (glass) rendering them more hydrophobic and hence resistant to adhesion by many surface contaminants. The mechanisms by which the necessary physical processes occur include direct momentum transfer, photodecomposition (chemical bond breaking and changing), photothermal effects and thermal expansion of the substrate and/or contaminants and/or assisting liquid/vapour layers.
Application of a pulsed DBD lamp by method of the invention to surface cleaning involves, depending on the particular application, a lamp which delivers the UV/VUV emission from a large area lamp (typically 5 cm2-10000 cm2, more typically 25-1000 cm2) onto a smaller area to be processed. The UV/VUV emission can be conditioned into a line source at the sample position by one-dimensional curvature or a spot source by two-dimensional curvatures of the UV/VUV pulsed DBD or a surrounding reflector. The sample to be processed is translated in the plane of the maximum power per unit arena. A nitrogen purge can be used in the volume in which the UV/VUV emission propagates. Threshold fluences for removal of micron and sub-micron particles from surfaces are typically 1 mJ/cm2-10 J/cm2, more typically 10 mJ/cm2-1 J/cm2, even more typically 50 mJ/cm2-400 mJ/cm2. Single pulse or multiple pulses can be used. More usually multiple pulses are required.
The cleaning efficiency increases with fluence above the threshold fluence The functional form of cleaning efficiency versus fluence depends on the spatial irradiance variation of the emission at the sample being processed The system may be housed in a vacuum chamber for some applications. Shorter wavelengths are in general more effective at cleaning surfaces (in the absence of any solvent assistance) but care must be taken to avoid any damage to the surface occurring in parallel with the cleaning, particularly at shorter wavelengths. Such cleaning of particulates has been affected in the prior art using pulsed laser sources.
One useful surface modification is the semi-permanent dehydroxylation of native silica glass surfaces. This can be affected with short pulse, high peak power UV/VUV emission from the invention. This can involve a geometry for the pulsed DBD lamp, or the system in which it is housed, which delivers the UV/VUV emission from a large area lamp onto a smaller area to be processed. The UV/VUV emission can be conditioned into a line source at the sample position by one-dimensional curvature or a spot source by two-dimensional curvatures of the UV/VUV pulsed DBD or a surrounding reflector. The sample to be processed is translated in the plane of the maximum power per unit area. A nitrogen purge can be used in the volume in which the UV/VUV emission propagates. The fluence at the processing sample is typically 1 mJ/cm2 to 1 J/cm2, more typically 10 mJ/cm2 to 500 mJ/cm2 and even more typically 100 mJ/cm2 to 200 mJ/cm2. The number of pulses of the emission that treat each area element of the sample (which is translated) is typically 1 to 106, more typically 10 to 105 and even more typically 100 to 104. The percentage of dehydroxylation (as determined from the ratio of SiOH+ to Si+ measured by time of flight secondary ion mass spectrometry (TOF SIMS)) is a function of both the fluence and number of pulses used. As a result of the treatment the sample is rendered more hydrophobic than native silica surfaces. Such dehydroxylation of silica glass surfaces has been affected in the prior art using UV pulsed laser sources. Photolithographic masking can be used to produce spatially patterned dehydroxylation.
Material etching/ablation applications (with moderate ablation threshold fluence) can be illustrated by polymer ablation using the method of the invention. Polymer (examples: PETG, poliimide, PET, PMMA) ablation has been affected in prior art by a variety of UV/VUV lamps and lasers The ablation/etching rates that can be affected by method of the invention cover most of the range of etch rates reported for AC DBD excimer lamps and UV pulsed lasers depending on whether the output from the invention is intensified as described above. Ablation/etch rates per pulse depend on fluence, pulse repetition frequency and material. Typical rates are between picometres per pulse and 0.1 xcexcm per pulse depending on whether the process proceeds sub-threshold or sup-threshold.